'\" et
.TH SENDMSG "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.\"
.SH NAME
sendmsg
\(em send a message on a socket using a message structure
.SH SYNOPSIS
.LP
.nf
#include <sys/socket.h>
.P
ssize_t sendmsg(int \fIsocket\fP, const struct msghdr *\fImessage\fP, int \fIflags\fP);
.fi
.SH DESCRIPTION
The
\fIsendmsg\fR()
function shall send a message through a connection-mode or
connectionless-mode socket. If the socket is a connectionless-mode
socket, the message shall be sent to the address specified by
.BR msghdr
if no pre-specified peer address has been set. If a peer address has
been pre-specified, either the message shall be sent to the address
specified in
.BR msghdr
(overriding the pre-specified peer address), or the function shall
return \-1 and set
.IR errno
to
.BR [EISCONN] .
If the socket is connection-mode, the destination address in
.BR msghdr
shall be ignored.
.P
The
\fIsendmsg\fR()
function takes the following arguments:
.IP "\fIsocket\fR" 12
Specifies the socket file descriptor.
.IP "\fImessage\fR" 12
Points to a
.BR msghdr
structure, containing both the destination address and the buffers for
the outgoing message. The length and format of the address depend on
the address family of the socket. The
.IR msg_flags
member is ignored.
.IP "\fIflags\fR" 12
Specifies the type of message transmission. The application may
specify 0 or the following flag:
.RS 12 
.IP MSG_EOR 14
Terminates a record (if supported by the protocol).
.IP MSG_OOB 14
Sends out-of-band data on sockets that support out-of-bound data. The
significance and semantics of out-of-band data are protocol-specific.
.IP MSG_NOSIGNAL 14
Requests not to send the SIGPIPE signal if an attempt to send is made
on a stream-oriented socket that is no longer connected. The
.BR [EPIPE] 
error shall still be returned.
.RE
.P
The
.IR msg_iov
and
.IR msg_iovlen
fields of
.IR message
specify zero or more buffers containing the data to be sent.
.IR msg_iov
points to an array of
.BR iovec
structures;
.IR msg_iovlen
shall be set to the dimension of this array. In each
.BR iovec
structure, the
.IR iov_base
field specifies a storage area and the
.IR iov_len
field gives its size in bytes. Some of these sizes can be zero. The
data from each storage area indicated by
.IR msg_iov
is sent in turn.
.P
Successful completion of a call to
\fIsendmsg\fR()
does not guarantee delivery of the message. A return value of \-1
indicates only locally-detected errors.
.P
If space is not available at the sending socket to hold the message to
be transmitted and the socket file descriptor does not have O_NONBLOCK
set, the
\fIsendmsg\fR()
function shall block until space is available. If space is not
available at the sending socket to hold the message to be transmitted
and the socket file descriptor does have O_NONBLOCK set, the
\fIsendmsg\fR()
function shall fail.
.P
If the socket protocol supports broadcast and the specified address is
a broadcast address for the socket protocol,
\fIsendmsg\fR()
shall fail if the SO_BROADCAST option is not set for the socket.
.P
The socket in use may require the process to have appropriate
privileges to use the
\fIsendmsg\fR()
function.
.SH "RETURN VALUE"
Upon successful completion,
\fIsendmsg\fR()
shall return the number of bytes sent. Otherwise, \-1 shall
be returned and
.IR errno
set to indicate the error.
.SH ERRORS
The
\fIsendmsg\fR()
function shall fail if:
.TP
.BR EAGAIN " or " EWOULDBLOCK
.br
The socket's file descriptor is marked O_NONBLOCK and the requested
operation would block.
.TP
.BR EAFNOSUPPORT
.br
Addresses in the specified address family cannot be used with this
socket.
.TP
.BR EBADF
The
.IR socket
argument is not a valid file descriptor.
.TP
.BR ECONNRESET
A connection was forcibly closed by a peer.
.TP
.BR EINTR
A signal interrupted
\fIsendmsg\fR()
before any data was transmitted.
.TP
.BR EINVAL
The sum of the
.IR iov_len
values overflows an
.BR ssize_t .
.TP
.BR EMSGSIZE
The message is too large to be sent all at once (as the socket
requires), or the
.IR msg_iovlen
member of the
.BR msghdr
structure pointed to by
.IR message
is less than or equal to 0 or is greater than
{IOV_MAX}.
.TP
.BR ENOTCONN
The socket is connection-mode but is not connected.
.TP
.BR ENOTSOCK
The
.IR socket
argument does not refer to a socket.
.TP
.BR EOPNOTSUPP
The
.IR socket
argument is associated with a socket that does not support one or more
of the values set in
.IR flags .
.TP
.BR EPIPE
The socket is shut down for writing, or the socket is connection-mode
and is no longer connected. In the latter case, and if the socket is of
type SOCK_STREAM or SOCK_SEQPACKET and the MSG_NOSIGNAL flag is not set,
the SIGPIPE signal is generated to the calling thread.
.P
If the address family of the socket is AF_UNIX, then
\fIsendmsg\fR()
shall fail if:
.TP
.BR EIO
An I/O error occurred while reading from or writing to the file
system.
.TP
.BR ELOOP
A loop exists in symbolic links encountered during resolution of the
pathname in the socket address.
.TP
.BR ENAMETOOLONG
.br
The length of a component of a pathname is longer than
{NAME_MAX}.
.TP
.BR ENOENT
A component of the pathname does not name an existing file or the path
name is an empty string.
.TP
.BR ENOTDIR
A component of the path prefix of the pathname in the socket address
names an existing file that is neither a directory nor a symbolic link
to a directory, or the pathname in the socket address contains at least
one non-\c
<slash>
character and ends with one or more trailing
<slash>
characters and the last pathname component names an existing file that
is neither a directory nor a symbolic link to a directory.
.br
.P
The
\fIsendmsg\fR()
function may fail if:
.TP
.BR EACCES
Search permission is denied for a component of the path prefix; or
write access to the named socket is denied.
.TP
.BR EDESTADDRREQ
.br
The socket is not connection-mode and does not have its peer address
set, and no destination address was specified.
.TP
.BR EHOSTUNREACH
.br
The destination host cannot be reached (probably because the host is
down or a remote router cannot reach it).
.TP
.BR EIO
An I/O error occurred while reading from or writing to the file system.
.TP
.BR EISCONN
A destination address was specified and the socket is already
connected.
.TP
.BR ENETDOWN
The local network interface used to reach the destination is down.
.TP
.BR ENETUNREACH
.br
No route to the network is present.
.TP
.BR ENOBUFS
Insufficient resources were available in the system to perform the
operation.
.TP
.BR ENOMEM
Insufficient memory was available to fulfill the request.
.P
If the address family of the socket is AF_UNIX, then
\fIsendmsg\fR()
may fail if:
.TP
.BR ELOOP
More than
{SYMLOOP_MAX}
symbolic links were encountered during resolution of the pathname in
the socket address.
.TP
.BR ENAMETOOLONG
.br
The length of a pathname exceeds
{PATH_MAX},
or pathname resolution of a symbolic link produced an intermediate
result with a length that exceeds
{PATH_MAX}.
.LP
.IR "The following sections are informative."
.SH "EXAMPLES"
Done.
.SH "APPLICATION USAGE"
The
\fIselect\fR()
and
\fIpoll\fR()
functions can be used to determine when it is possible to send more
data.
.SH "RATIONALE"
None.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "\fIgetsockopt\fR\^(\|)",
.IR "\fIpoll\fR\^(\|)",
.IR "\fIpselect\fR\^(\|)",
.IR "\fIrecv\fR\^(\|)",
.IR "\fIrecvfrom\fR\^(\|)",
.IR "\fIrecvmsg\fR\^(\|)",
.IR "\fIsend\fR\^(\|)",
.IR "\fIsendto\fR\^(\|)",
.IR "\fIsetsockopt\fR\^(\|)",
.IR "\fIshutdown\fR\^(\|)",
.IR "\fIsocket\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "\fB<sys_socket.h>\fP"
.\"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
.PP
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
